Winds are starting to strengthen in the capital as storm force gales and heavy rain are expected to lash parts of New Zealand devastated by this morning's quake.

Severe weather warnings are in force as the weather begins to deteriorate across the top of the South Island and lower North Island.

MetService meteorologist Tom Adams said winds of up to 140km/h were expected to hurtle across Cook Strait this afternoon before moving into Wellington tonight.

Adams said the winds would be potentially treacherous given the number of unsecured structures around the capital.

"One of the big impacts will be the winds in Wellington because of the damage in the central business district."

Fortunately, Kaikoura would be spared the worst of the wind and rain.

Heavy rain was forecast for the north-west corner of the South Island with people told to watch for coastal flooding, especially around Nelson and the north-west of the South Island.

Adams said a front was expected to cross the South Island from late today preceded by heavy, thundery rain and northerly gales. High tides co-inciding with heavy rainfall and strong winds were likely to pose problems for some exposed coastal areas.

A rain warning was in force for northwest Nelson, Buller and Westland north of Fox Glacier. Up to 200mm of rain was expected to fall about the ranges and 150mm near the coast.

A heavy rain watch was in force across Nelson, the western ranges of Marlborough and North Canterbury high country.

In the North Island rain was expected to become heavy tonight with the heaviest falls expected between 3am and midday Tuesday. Rain warnings have also been issued for Mt Taranaki and the headwaters of Whanganui River.

A severe weather warning has been issued for strong winds with damaging gusts up to 140km/h in Wellington, Wairarapa and Marlborough from tonight into Tuesday morning.